This invention relates to voice recording by differential vector quantization.
The market for voice recording and reproducing devices, often referred to as voice recorders, is now in a state of active growth. The reason is that a combination of increasing record/playback time and decreasing cost is opening up new applications in business tools and consumer electronic devices. In particular, digital voice recorders employing integrated-circuit (IC) memory as storage media are now finding many applications.
For business applications, a long recording time and good sound quality are essential requirements. The factor enabling these requirements to be met has been the recent rapid progress in high-efficiency compression technology. Compression is achieved through coding techniques that make intensive use of complex, sophisticated digital signal processing, which requires a fast, high-performance digital signal processor (DSP). For that reason, business-grade voice recorders based on IC memory still tend to be fairly expensive.
For consumer products such as radio sets, long recording time and good sound quality are secondary considerations; the essential requirement is low cost. Applications in consumer products must dispense with complex, sophisticated signal processing and employ coding techniques that can be implemented comparatively simply.
Vector quantization (VQ) is one such technique. Briefly, in vector quantization, a voice waveform is divided into short frames, each of which is approximated by a pattern taken from a codebook, and index numbers identifying the patterns are recorded in place of the actual waveform data. Differential vector quantization is a similar technique that predicts the voice waveform in each frame and uses the patterns in the codebook to approximate the difference between the predicted and actual waveforms.
While vector quantization has the advantage of simplicity, it may require a large codebook to achieve satisfactory sound quality. Differential vector quantization can provide equivalent sound quality with a smaller codebook, but requires an extra prediction step. In conventional differential vector quantization, the cost of the prediction process is fairly high, because it involves multiplication of a full frame of waveform data by a matrix of prediction coefficients. The cost is a computational cost if the prediction is done by software, or a physical circuit cost if the prediction is done by hardware. In either case, there is an associated economic penalty: more circuitry is required, or a faster processor is required.
Further details will be given in the detailed description of the invention.